Methods of transmitting and receiving frame by station operating in power save mode in wireless lan system and apparatus for supporting same

ABSTRACT

The present invention relates to methods of transmitting and receiving a frame, which are performed by a station operating in a power save mode in a wireless LAN system. The method includes: transmitting a poll frame requesting the transmission of a buffered frame to an access point AP, wherein the poll frame includes a sustain time field that indicates a service section; and receiving at least one buffered frame from the AP within the sustain time in response to the poll frame. A method is provided for transmitting and receiving a frame executed by a station (STA) operating in a power sub mode in a wireless LAN system. The method includes transmitting a first poll frame requesting the transmission of a buffered frame to an access point AP, receiving an acknowledgement ACK frame in response to the first poll frame, and receiving at least one buffered frame from the AP.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a wireless LAN system, and morespecifically, a frame transmission/reception method by a stationoperating in a power save mode in a wireless LAN system and an apparatussupporting the same.

Related Art

With the growth of information communication technology, variouswireless communication technologies have been recently developed. Amongothers, wireless LAN (Local Area Network) is a technology that allowsfor wireless access to the Internet at home or business or in a specificservice area using a handheld terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

IEEE 802.11n is a technology standard that has been recently establishedin order to overcome the limit to communication speed that has beenrecognized as a weakness of wireless LAN. IEEE 802.11n aims to increasenetwork speed and reliability and expand coverage of a wireless network.More specifically, the IEEE 802.11n system adopts MIMO (Multiple Inputsand Multiple Outputs) technology that uses multiple antennas at both atransmission unit and a reception unit thereof so as to optimize dataspeed and to minimize transmission errors while supporting a highthroughput (HT) of data processing speed up to 540 Mbps.

In the wireless LAN system, a station (STA) supports a power save mode.The station may prevent unnecessary power consumption by entering into adoze state. In case there is traffic associated with data that intendsto be sent to an STA that is operating in a doze state, an access point(AP) may notify this to the STA. The STA recognizes existence of trafficassociated with data intended to be sent thereto and may request thatthe AP be sending it to the STA. The AP may transmit a frame in responseto the STA's request.

Meanwhile, if the AP may transmit only one frame in response to therequest from the STA that has entered into an awake state, it may beinefficient in view of traffic processing. Further, the STA shiftsbetween the awake state and doze state more frequently, and thusefficiency may be deteriorated in terms of power saving operation.Accordingly, a need exists for a frame transmission and reception methodthat may enhance power save mode efficiency of an STA and good trafficprocessing.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a frame transmissionand reception method that is performed by a station (STA) operating in apower save mode in a wireless local area network system and an apparatussupporting the same.

In an aspect, a method of transmitting and receiving a frame, performedby a station (STA) operating in a power save mode in a wireless localarea network system is provided. The method includes transmitting, to anaccess point (AP), a poll frame for requesting transmission of abuffered frame, wherein the poll frame includes a duration fieldindicating a service period and receiving, from the AP, at least onebuffered frame during the service period in response to the poll frame.

The method may further include transmitting an acknowledgement (ACK)frame as a response to acknowledge an reception of the at least onebuffered frame.

The method may further include transmitting the ACK frame before theservice period is terminated.

The ACK frame may be transmitted corresponding to a last buffered frameamong the at least one buffered frame.

The method may further include transmitting, to the AP, a pre-poll framefor requesting transmission of the buffered frame, receiving, from theAP, an acknowledgement (ACK) frame in response to the pre-poll frame,and entering into a doze state after receiving the ACK frame.

The pre-poll frame may include a polled service period interval field,and the polled service period interval field may include informationrelated to a time when the STA transmits the poll frame.

The method may further include entering into an awake state at a timeindicated by the polled service period interval field, and performingcontention for channel access. The poll frame is transmitted when achannel access authority is obtained through the contention.

In another aspect, a wireless device for operating in a wireless localarea network system is provided. The wireless device includes atransceiver transmitting and receiving a radio signal, and a processoroperatively coupled with the transceiver and configured to transmit, toan access point (AP), a poll frame for requesting transmission of abuffered frame, wherein the poll frame includes a duration fieldindicating a service period, and receive from the AP at least onebuffered frame during the service period in response to the poll frame.

In still another aspect, a method of transmitting and receiving a frame,performed by a station (STA) operating in a power save mode in awireless local area network system is provided. The method includetransmitting, to an access point (AP), a first poll frame for requestingtransmission of a buffered frame, receiving an acknowledgement (ACK)frame in response to the first poll frame, and receiving at least onebuffered frame from the AP.

The ACK frame may include polled service period information related to atime when the AP starts transmission of the at least one buffered frame.

if the polled service period information indicates that an immediatelybuffered frame is to be transmitted, the at least one buffered frame maybe received an SIFS (Short Inter Frame Space) after receiving the ACKframe, and the method may further include entering into a doze stateafter receiving the at least one buffered frame.

If the polled service period information indicates a specific time whenthe buffered frame is to be transmitted, the method may further includeentering into a doze state after receiving the ACK frame, entering intoan awake state at a time indicated by the polled service periodinformation, transmitting to the AP a second poll frame for requestingtransmission of the at least one buffered frame, and receiving the atleast one buffered frame in response to the second poll frame.

The method may further include entering into a doze state afterreceiving the at least one buffered frame.

The second poll frame may include a duration field. The duration fieldmay indicate a service period. The at least one or more buffered framemay be transmitted during the service period.

By a frame transmission and reception method according to an embodimentof the present invention, an STA may receive a buffered frame from an APduring a polled service period over multiple times and may enter into adoze state between polled service periods and operate, so that powerconsumption can be prevented. Further, the STA may receive at least oneor more buffered frames during one polled service period, thus enablingefficient data transmission and reception. In addition, the AP maytransmit a buffered frame during a service period even without RTS/CTSexchange in order to transmit a buffered frame, thus enhancing frametransmission and reception efficiency.

By a frame transmission and reception method according to an embodimentof the present invention, an STA may control a polled service periodaccording to a transmission state of an AP's buffered frame. This mayprevent the phenomenon that as a polled service period initiated by anSP-poll frame is unnecessarily maintained, even when transmission of aframe transmitted from the AP is not actually needed, a channel keepsbeing unnecessarily occupied since the STA holds channel accessauthority. Further, other STAs located in the service coverage of the APand/or STA may also obtain a channel access authority by adjusting anNAV according to an actually adjusted service period. Accordingly, theoverall throughput of the wireless LAN system may be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating the configuration of a general wirelessLAN (Local Area Network) system to which an embodiment of the presentinvention may apply.

FIG. 2 is a view illustrating a physical layer architecture of awireless LAN system supported by IEEE 802.11.

FIGS. 3 and 4 are block diagrams illustrating the format of a PPDU usedin a wireless LAN system to which an embodiment of the present inventionmay apply.

FIG. 5 is a block diagram illustrating the format of an MAC frameprovided in a wireless LAN system.

FIG. 6 is a block diagram illustrating the format of an HT controlfield.

FIG. 7 is a block diagram illustrating the format of an HT variantmiddle field for HT.

FIG. 8 is a block diagram illustrating the format of an HT variantmiddle field for VHT.

FIG. 9 is a view illustrating an example of power management operation.

FIG. 10 is a block diagram illustrating an example of a TIM elementformat.

FIG. 11 is a view illustrating an example of a bitmap control field anda partial virtual bitmap field according to an embodiment of the presentinvention.

FIG. 12 is a flowchart illustrating an example of an AP's respondingprocedure in a TIM protocol.

FIG. 13 is a flowchart illustrating another example of an AP'sresponding procedure in a TIM protocol.

FIG. 14 is a flowchart illustrating a procedure of a TIM protocol by aDTIM.

FIG. 15 is a view illustrating an example method of transmitting andreceiving a frame based on a TIM protocol and U-APSD.

FIG. 16 is a block diagram illustrating an MAC frame format of anSP-poll frame according to an embodiment of the present invention.

FIG. 17 is a view illustrating an example method of transmitting andreceiving a frame by an STA operating in a power save mode according toanother embodiment of the present invention.

FIG. 18 is a view illustrating another example of a method oftransmitting and receiving a frame by an STA operating in a power savemode according to another embodiment of the present invention.

FIG. 19 is a view illustrating another example of a method oftransmitting and receiving a frame by an STA operating in a power savemode according to an embodiment of the present invention.

FIG. 20 is a view illustrating another example method of transmittingand receiving a frame according to an embodiment of the presentinvention.

FIG. 21 is a flowchart illustrating another example method oftransmitting and receiving a frame according to an embodiment of thepresent invention.

FIG. 22 is a block diagram illustrating the format of a polled SPinterval information element according to an embodiment of the presentinvention.

FIG. 23 is a block diagram illustrating the format of a response timeinformation element according to an embodiment of the present invention.

FIG. 24 is a view illustrating an example method of transmitting andreceiving a frame by an STA operating in a power save mode according toanother embodiment of the present invention.

FIG. 25 is a view illustrating another example method of transmittingand receiving a frame by an STA operating in a power save mode accordingto another embodiment of the present invention.

FIG. 26 is a block diagram illustrating a wireless device in which anembodiment of the present invention may be implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a view illustrating the configuration of a general wirelessLAN (Local Area Network) system to which an embodiment of the presentinvention may apply.

Referring to FIG. 1, the wireless LAN system includes one or more basicservice sets (BSSs). A BSS is a set of stations (STAs) that may besuccessfully synchronized with each other and may communicate with eachother, and is not a concept indicating a specific area.

An infrastructure BSS includes one or more non-AP stations (non-APSTA1(21), non-AP STA2(22), non-AP STA3(23), non-AP STA4(24), and non-APSTAa (30)), an AP (Access Point) 10 providing a distribution service,and a distribution system (DS) linking multiple APs. In theinfrastructure BSS, the AP manages the non-AP STAs of the BSS.

In contrast, an independent BSS (IBSS) is a BSS operating in an ad-hocmode. The IBSS does not include an AP and thus lacks a centralizedmanagement entity. That is, in the IBSS, non-AP STAs are managed in adistributed manner. In the IBSS, all the STAs may be mobile STAs, anddue to no permission to access the DS, constitute a self-containednetwork.

The STA is any functional medium that includes a medium access control(MAC) that follows the IEEE (Institute of Electrical and ElectronicsEngineers) 802.11 standards and a physical layer interface of a radiomedium and in broader concept includes an AP and a non-AP station.

The non-AP STA is an STA, but not an AP, and may also be referred to asa mobile terminal, wireless device, wireless transmit/receive unit(WTRU), user equipment (UE), mobile station (MS), mobile subscriber unitor simply user. Hereinafter, for ease of description, the non-AP STA isdenoted STA.

The AP is a functional medium that provides access to a DS via a radiomedium for an STA associated with an AP. In an infrastructure BSSincluding an AP, communication between STAs is in principle achieved viaan AP, but in case a direct link is set up, the STAs may perform directcommunication between each other. The AP may also be referred to as acentral controller, base station (BS), node-B, BTS (Base TransceiverSystem), site controller, or managing STA.

A plurality of BSSs including the BSS shown in FIG. 1 may be connectedto each other via a distribution system (DS). The plurality of BSSslinked with each other through a DS is referred to as an extendedservice set (ESS). The APs and/or STAs included in the ESS maycommunicate with each other, and in the same ESS, STAs may travel fromone BSS to another BSS while maintaining seamless communication.

In the wireless LAN system according to IEEE 802.11, the basic accessmechanism of MAC (Medium Access Control) is the CSMA/CS (Carrier SenseMultiple Access with Collision Avoidance) mechanism The CSMA/CSmechanism is also referred to as distributed coordination function (DCF)of IEEE 802.11 MAC, and basically, it adopts a “listen before talk”access mechanism. Following such type of access mechanism, an AP and/orSTA senses a radio channel or medium prior to transmission. If as aresult of the sensing, the medium is determined to be in idle state,frame transmission is initiated through the medium. On the contrary, ifthe medium is sensed to be in occupied state, the AP and/or STA sets adeferred time for medium access and waits without starting its owntransmission.

The CSMA/CS mechanism includes virtual carrier sensing in addition tophysical carrier sensing in which an AP and/or STA directly senses amedium. The virtual carrier sensing is to make up for a problem that mayoccur in connection with medium access, such as hidden node problem. Inorder for virtual carrier sensing, the MAC of the wireless LAN systemmakes use of a network allocation vector (NAV). The NAV is a value bywhich an AP and/or STA currently using a medium or having authority touse the medium informs other AP and/or STA of a time remaining until themedium turns available. Accordingly, the value set by the NAVcorresponds to a period during which the use of the medium is scheduledby the AP and/or STA transmitting a frame.

The IEEE 802.11 MAC protocol, together with a DCF, offers an HCF (HybridCoordination Function) that is based on a PCF (Point CoordinationFunction) that periodically performs polling so that all receiving APsand/or STAs may receive data packets in polling-based synchronizedaccess scheme with the DCF. The HCF has HCCA (HCF Controlled ChannelAccess) that uses contention free-based channel access scheme using apolling mechanism and EDCA (Enhanced Distributed Channel Access) thathas a contention-based access scheme for providing data packets tomultiple users. The HCF includes a medium access mechanism for enhancingQoS (Quality of Service) of wireless LAN and may transmit QoS data inboth a contention period (CP) and contention free period (CFP).

The wireless communication system cannot be immediately aware of theexistence of a network due to the characteristics of the radio mediumwhen an STA powers on and starts operating. Accordingly, in order toaccess a network, an STA, whatever type it is, should go through anetwork discovery process. When discovering a network through thenetwork discovery process, the STA selects a network to subscribe tothrough a network selection process. Thereafter, the STA subscribes tothe selected network and performs data exchange at a transmissionend/reception end.

In the wireless LAN system, the network discovery process is implementedas a scanning procedure. The scanning procedure is separated intopassive scanning and active scanning The passive scanning is achievedbased on a beacon frame that is periodically broadcast by an AP. Ingeneral, an AP in the wireless LAN system broadcasts a beacon frame at aspecific interval (for example, 100 msec). The beacon frame includesinformation on a BSS managed by it. The STA passively awaits receptionof the beacon frame at a specific channel When obtaining the informationon the network by receiving the beacon frame, the STA terminates thescanning procedure at the specific channel The STA need not transmit aseparate frame in achieving passive scanning, and the passive scanningis rather done once the beacon frame is received. Accordingly, thepassive scanning may reduce the overall overhead. However, it suffersfrom a scanning time that is increased in proportion to the transmissionperiod of the beacon frame.

The active scanning is that the STA actively broadcasts a probe requestframe at a specific channel to request that all the APs to receive theprobe request frame send network information to the STA. When receivingthe probe request frame, an AP waits for a random time so as to preventframe collision, and then includes network information in a proberesponse frame, then transmits the probe response frame to the STA. TheSTA receives the probe response frame to thereby obtain the networkinformation, and the scanning procedure is then ended. The activescanning may get scanning done relatively quickly, but may increase theoverall network overhead due to the need of a frame sequence that comesfrom request-response.

When finishing the scanning procedure, the STA selects a network per aspecific standard on itself and then performs an authenticationprocedure alongside the AP. The authentication procedure is achieved in2-way handshake. When completing the authentication procedure, the STAproceeds with an association procedure together with the AP.

The association procedure is performed in two-way handshake. First, theSTA sends an association request frame to the AP. The associationrequest frame includes information on the STA's capabilities. Based onthe information, the AP determines whether to allow association with theSTA. When determining whether to allow association, the AP transmits anassociation response frame to the STA. The association response frameincludes information indicating whether to allow association andinformation indicating the reason for association being allowed orfailing. The association response frame further includes information oncapabilities supportable by the AP. In case association is successfullydone, normal frame exchange is done between the AP and STA. In caseassociation fails, the association procedure is retried based on theinformation on the reason for the failure included in the associationresponse frame or the STA may send a request for association to otherAP.

In order to overcome limit to speed that is considered to be a weaknessin wireless LAN, IEEE 802.11n has been established relatively in recentyears. IEEE 802.11n aims to increase network speed and reliability whileexpanding wireless network coverage. More specifically, IEEE 802.11nsupports high throughput (HT) that reaches data processing speed up to540 Mbps and is based on MIMO (Multiple Inputs and Multiple Outputs)technology that adopts multiple antennas at both transmission end andreception end in order to optimize data speed and minimize transmissionerrors.

As wireless LAN spreads and more diversified applications using wirelessLAN show up, a need for a new wireless LAN system arises for supportinga higher throughput than the data processing speed supported by IEEE802.11n. The wireless LAN system supporting very high throughput (VHT)is a subsequent version of the IEEE 802.11n wireless LAN system, whichis a new one recently suggested to support a throughput more than 500Mbps for a single user and data processing speed more than 1 Gpbs formultiple users in an MAC service access point (SAP).

Advancing further than the existing wireless LAN system supporting 20MHz or 40 MHz, the VHT wireless LAN system intends to support 80 MHz,contiguous 160 MHz, non-contiguous 160 MHz band transmission and/or morebandwidth transmission. Further, the VHT wireless LAN system supports250QAM that is more than a maximum of 64QAM (Quadrature AmplitudeModulation) of the existing wireless LAN system.

Since the VHT wireless LAN system supports an MU-MIMO (MultiUser-Multiple Input Multiple Output) transmission method for higherthroughput, the AP may transmit a data frame simultaneously to at leastone or more MIMO-paired STAs. The number of paired STAs may be maximally4, and when the maximum number of spatial streams is eight, each STA maybe assigned up to four spatial streams.

Referring back to FIG. 1, in the wireless LAN system shown in thefigure, the AP 10 may simultaneously transmit data to an STA groupincluding at least one or more STAs among a plurality of STAs 21, 22,23, 24, and 30 associated with the AP 10. In FIG. 1, by way of example,the AP conducts MU-MIMO transmission to the STAs. However, in a wirelessLAN system supporting TDLS (Tunneled Direct Link Setup) or DLS (DirectLink Setup) or mesh network, an STA to transmit data may send a PPDU toa plurality of STAs using an MU-MIMO transmission scheme. Hereinafter,an example where an AP transmits a PPDU to a plurality of STAs accordingto an MU-MIMO transmission scheme is described.

Data may be transmitted through different spatial streams to each STA.The data packet transmitted by the AP 10 may be referred to as a PPDU,which is generated at the physical layer of the wireless LAN system andtransmitted, or a frame as a data field included in the PPDU. That is,the PPDU for SU (single user)-MIMO and/or MU-MIMO or data field includedin the PPDU may be called a MIMO packet. Among them, the PPDU for MUsmay be called an MU packet. In the example of the present invention,assume that a transmission target STA group MU-MIMO-paired with the AP10 includes STA1 21, STA2 22, STA3 23, and STA4 24. At this time, nospatial stream is assigned to a specific STA in the transmission targetSTA group, so that no data may be transmitted to the specific STA.Meanwhile, assume that STAa 30 is associated with the AP but is notincluded in the transmission target STA group.

In the wireless LAN system, an identifier may be assigned to thetransmission target STA group in order to support MU-MIMO transmission,and this identifier is denoted group ID. The AP sends a group IDmanagement frame including group definition information for allocatinggroup IDs to the STAs supporting MU-MIMO transmission and accordinglythe group IDs are assigned to the STAs before PPDU transmission. One STAmay be assigned a plurality of group IDs.

Table 1 below represents information elements included in the group IDmanagement frame.

TABLE 1 order information 1 category 2 VHT action 3 Membership status 4Spatial stream position

The category field and VHT action field are configured so that the framecorresponds to a management frame and to be able to identify being agroup ID management frame used in a next-generation wireless LAN systemsupporting MU-MIMO.

As in Table 1, the group definition information includes membershipstatus information indicating whether to belong to a specific group ID,and in case of belonging to the group ID, information indicating thenumber of position to which the spatial stream set of the STAcorresponds in all the spatial streams according to MU-MIMOtransmission.

Since one AP manages a plurality of group IDs, the membership statusinformation provided to one STA needs to indicate whether the STAbelongs to each of the group IDs managed by the AP. Accordingly, themembership status information may be provided in the form of an array ofsubfields indicating whether it belongs to each group ID. The spatialstream position information indicates the position of each group ID, andthus, may be provided in the form of an array of subfields indicatingthe position of a spatial stream set occupied by the STA with respect toeach group ID. Further, the membership status information and spatialstream position information for one group ID may be implemented in onesubfield.

The AP, in case of sending a PPDU to a plurality of STAs through anMU-MIMO transmission scheme, transmits the PPDU, with informationindicating a group identifier (group ID) in the PPDU as controlinformation. When receiving the PPDU, an STA verifies whether it is amember STA of the transmission target STA group by checking the group IDfield. If the STA is a member of the transmission target STA group, theSTA may identify what number of position where the spatial stream settransmitted to the STA is located in the entire spatial stream. The PPDUincludes information on the number of spatial streams allocated to thereceiving STA, and thus, the STA may receive data by discovering thespatial streams assigned thereto.

Meanwhile, TV WS (White Space) draws attention as a newly availablefrequency band in the wireless LAN system. TV WS refers to an unusedfrequency band that is left as the analog TV broadcast is digitalized inthe U.S. For example, TV WS includes a 54 to 598 MHz band. However, thisis merely an example, and TV WS may be a permitted band that may befirst used by a licensed user. The licensed user means a user that ispermitted for use of a permitted band, and may also be referred to as alicensed device, primary user, or incumbent user.

The AP and/or STA operating in the TV WS should offer a protectionfunction as to a licensed user, and this is because a licensed user haspriority as to use of a TV WS band. For instance, in case a licenseduser such as a microphone is already using a specific WS channel that isa frequency band split per protocol to have a certain bandwidth in theTV WS band, the AP and/or STA cannot use the frequency bandcorresponding to the WS channel in order to protect the licensed user.Further, the AP and/or STA should stop use of the frequency band if thelicensed user happens to use the frequency band that is being used fortransmission and/or reception of a current frame.

Accordingly, the AP and/or STA should first grasp whether a specificfrequency band in the TV WS band is available, in other words, whetherthere is a licensed user in the frequency band. Grasping whether thereis a licensed user in the specific frequency band is denoted spectrumsensing. As a spectrum sensing mechanism, an energy detection scheme orsignature detection scheme may be utilized. If the strength of areceived signal is higher than a predetermined value, it is determinedthat it is being used by a licensed user, or if a DTV preamble isdetected, it may be determined to be being used by a licensed user.

FIG. 2 is a view illustrating a physical layer architecture of awireless LAN system supported by IEEE 802.11.

The IEEE 802.11 physical (PHY) architecture includes a PLME (PHY LayerManagement Entity), a PLCP (Physical Layer Convergence Procedure)sublayer 210, and a PMD (Physical Medium Dependent) sublayer 200. ThePLME provides a function of managing the physical layer in cooperationwith the MLME (MAC Layer Management Entity). The PLCP sublayer 210delivers an MPDU (MAC Protocol Data Unit) received from the MAC sublayer220 to the PMD sublayer in response to an instruction of the MAC 9layerbetween the MAC sublayer 220 and the PMD sublayer 200 or delivers aframe coming from the PMD sublayer 200 to the MAC sublayer 220. The PMDsublayer 200 is a PLCP lower layer and enables transmission andreception of a physical layer entity between two stations through aradio medium. The MPDU delivered by the MAC sublayer 220 is denoted aPSDU (Physical Service Data Unit) in the PLCP sublayer 210. The MPDU issimilar to the PSDU, but in case an A-MPDU (aggregated MPDU) obtained byaggregating a plurality of MPDUs is delivered, each MPDU may bedifferent from each PSDU.

The PLCP sublayer 210 adds an additional field including informationneeded by a physical layer transceiver while delivering a PSDU from theMAC sublayer 220 to the PMD sublayer 200. At the time, the added fieldmay include a PLCP preamble to the PSDU, a PLCP header, or tail bitsnecessary for turning a convolution encoder back into the zero state.The PLCP sublayer 210 receives from the MAC sublayer a TXVECTORparameter including control information necessary to generate andtransmit a PPDU and control information necessary for the STA to receiveand analyze a PPDU. The PLCP sublayer 210 uses information included inthe TXVECTOR parameter in generating a PPDU including the PSDU.

The PLCP preamble plays a role to let the receiver prepare for asynchronization function and antenna diversity before the PSDU istransmitted. The data field may include padding bits to the PSDU, aservice field including a bit sequence for initializing a scrambler, anda coded sequence where the tail bits-added bit sequence is encoded. Atthe time, as an encoding scheme, depending on the encoding schemesupported by the STA receiving the PPDU, BCC (Binary Convolution Coding)encoding or LDPC (Low Density Parity Check) encoding may be selected.The PLCP header includes a field including information on the PPDU (PLCPProtocol Data Unit) to be transmitted, and this will be described infurther detail below with reference to FIGS. 3 and 4.

The PLCP sublayer 210 adds the above-described fields to the PSDU tothereby generate a PPDU (PLCP Protocol Data Unit) and transmits the PPDUto a receiving station via the PMD sublayer, and the receiving STAreceives the PPDU and obtains the information necessary for restoringdata from the PLCP preamble and PLCP header and restores data. The PLCPsublayer of the receiving station delivers to the MAC sublayer theRXVECTOR parameter including the control information contained in thePLCP header and the PLCP preamble and may analyze the PPDU and obtaindata in the receiving state

FIGS. 3 and 4 are block diagrams illustrating the format of a PPDU usedin a wireless LAN system to which an embodiment of the present inventionmay apply. Hereinafter, the STA operating in a legacy wireless LANsystem based on IEEE 802.11a/b/g, existing wireless LAN standards priorto IEEE 802.11n is referred to a legacy STA (L-STA). Further, the STAthat may support HT in an HT wireless LAN system based on IEEE 802.11nis referred to as an HT-STA.

Subfigure (a) of FIG. 3 illustrates the format of a legacy PPDU (L-PPDU)used in IEEE 802.11a/b/g that are existing wireless LAN system standardsbefore IEEE 802.11n. Accordingly, in the HT wireless LAN system to whichthe IEEE 802.11n standard applies, the legacy-STA (L-STA) may transmitand receive an L-PPDU having the same format.

The L-PPDU 310 includes an L-STF 311, an L-LTF 312, an L-SIG field 313,and a data field 314.

The L-STF 311 is used for frame timing acquisition, AGC (Automatic GainControl) convergence, and coarse frequency acquisition.

The L-LTF 312 is used for frequency offset and channel estimation.

The L-SIG field 313 includes control information for demodulating anddecoding the data field 314.

In the L-PPDU, the L-STF 311, the L-LTF 312, the L-SIG field 313, andthe data field 314 may be transmitted in the order thereof.

Subfigure (b) of FIG. 3 is a block diagram illustrating an HT-mixed PPDUformat that enables an L-STA and an HT-STA to co-exist. The HT-mixedPPDU 320 includes an L-STF 321, an L-LTF 322, an L-SIG 3 field 23, anHT-SIG field 324, an HT-STF 325, and a plurality of HT-LTFs 326, and adata field 327.

The L-STF 321, L-LTF 322, and L-SIG field 323 are the same as thosedenoted by reference numerals 311, 312, and 313, respectively.Accordingly, the L-STA, even when receiving the HT-mixed PPDU 320, mayanalyze the data field through the L-STF 321, L-LTF 322, and L-SIG 323.However, the L-SIG 323 may further include information for channelestimation that is to be conducted for the HT-STA to receive theHT-mixed PPDU 320 and to decipher the L-SIG 323, HT-SIG 324, and HT-STF325.

The HT-STA may be aware that the HT-mixed PPDU 320 is a PPDU for itselfthrough the HT-SIG 324 coming after the L-SIG 323, and based on this,may demodulate and decode the data field 327.

The HT-STF 325 may be used for frame timing synchronization or AGCconvergence for an HT-STA.

The HT-LTF 326 may be used for channel estimation to demodulate the datafield 327. Since IEEE 802.11n supports SU-MIMO, there may be a pluralityof HT-LTFs 326 for each data field transmitted in a plurality of spatialstreams.

The HT-LTF 326 may consist of a data HT-LTF used for channel estimationfor a spatial stream and an extension HT-LTF additionally used for fullchannel sounding. Accordingly, the number of the plurality of HT-LTFs326 may be equal to or more than the number of spatial streamstransmitted.

In the HT-mixed PPDU 320, the L-STF 321, L-LTF 322, and the L-SIG field323 are first transmitted so that the L-STA may also receive it tothereby obtain data. Thereafter, the HT-SIG field 324 is transmitted fordemodulating and decoding data transmitted for the HT-STA.

The HT-SIG field 324 and its precedents are transmitted withoutbeamforming, so that the L-STA and the HT-STA may receive the PPDU tothereby obtain data, and the HT-STF 325, HT-LTF 326 and the data field327 transmitted thereafter are subjected to radio signal transmissionthrough precoding. Here, the HT-STF 325 is transmitted and then theplurality of HT-LTFs 326 and the data field 327 are transmitted so thata power variation by precoding may be taken into account by the STAconducting reception through precoding.

Although in the HT wireless LAN system, the HT-STA using 20 MHz uses 52data subcarriers per OFDM symbol, the L-STA using the same frequency, 20MHz, still makes use of 48 subcarriers per OFDM symbol. In order forbackward compatibility with the existing systems, the HT-SIG field 324in the HT-mixed PPDU 320 is decoded using the L-LTF 322, so that theHT-SIG field 324 is constituted of 48×2 data subcarriers. Thereafter,the HT-STF 325 and the HT-LTF 326 consists of 52 data subcarriers perOFDM symbol. As a result, the HT-SIG field 324 is supported with ½, BPSK(Binary Phase Shift Keying), each HT-SIG field 324 consists of 24 bits,and is thus transmitted with a total of 48 bits. In other words, channelestimation for the L-SIG field 323 and the HT-SIG field 324 utilizes theL-LTF 322, and the bit stream constituting the L-LTF 322 is representedas in Equation 1 below. The L-LTF 322 consists of 48 data subcarriersexcept a DC subcarrier per symbol.

L_(−26,26)={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}  [Equation 1]

Subfigure (c) of FIG. 3 is a block diagram illustrating an HT-greenfieldPPDU 330 format that may be used only by an HT-STA. The HT-GF PPDU 330includes an HT-GF-STF 331, an HT-LTF1 332, an HT-SIG 333, a plurality ofHT-LTF2's 334, and a data field 335.

The HT-GF-STF 331 is used for frame timing acquisition and AGC.

The HT-LTF1 332 is used for channel estimation.

The HT-SIG 333 is used for demodulating and decoding the data field 335.

The HT-LTF2 334 is used for channel estimation for demodulating the datafield 335. Likewise, the HT-STA uses SU-MIMO and thus requires channelestimation for each data field transmitted I a plurality of spatialstreams. Accordingly, a plurality of HT-LTFs 326 may be configured.

The plurality of HT-LTF2's 334 may consist of a plurality of extensionHT-LTFs and a plurality of data HT-LTFs like the HT-LTFs 326 of theHT-mixed PPDU 320.

Each of the data fields 314, 327, and 335 may include a service field, ascrambled PSDU, a tail bit and a padding bit. The service field may beused for initializing a scrambler. The service field may be configuredas 16 bits. In such case, seven bits may be configured for initializinga scrambler. The tail field may be configured as a bit sequencenecessary for turning a convolution encoder back into a zero state. Thetail field may be assigned a bit size that is proportional with thenumber of BCC (Binary Convolutional Code) encoders used for encodingdata to be transmitted. More specifically, it may be configured to havesix bits per BCC count.

FIG. 4 is a view illustrating an example of a PPDU format used in awireless LAN system supporting VHT.

Referring to FIG. 4, the PPDU 400 may include an L-STF 410, an L-LTF420, an L-SIG field 430, a VHT-SIGA field 440, a VHT-STF 450, a VHT-LTF460, a VHT-SIGB field 470, and a data field 480.

The PLCP sublayer configuring the PHY adds necessary information to thePSDU delivered from the MAC layer to generate the data field 480, addsto it the L-STF 410, the L-LTF 420, the L-SIG field 430, the VHT-SIGAfield 440, the VHT-STF 450, the VHT-LTF 460, and the VHT-SIGB field 470or other fields to thereby generate the PPDU 400, and transmits it toone or more STAs through the PMD sublayer constituting the PHY. Thecontrol information necessary for the PLCP sublayer to generate the PPDUand the control information that is included in the PPDU and transmittedto be used for the receiving STA to interpret the PPDU are provided fromthe TXVECTOR parameter delivered from the MAC layer.

The L-STF 410 is used for frame timing acquisition, AGC (Automatic GainControl) convergence, and coarse frequency acquisition.

The L-LTF 420 is used for channel estimation to demodulate the L-SIGfield 430 and the VHT-SIGA field 440.

The L-SIG field 430 is used for the L-STA to receive the PPDU 400 andinterpret the PPDU 400 to thereby obtain data. The L-SIG field 430includes a rate subfield, a length subfield, a parity bit and a tailfield. The rate subfield is set with a value indicating a bit rate fordata to be currently transmitted.

The length subfield is set as a value indicating the octet length of thePSDU by which the MAC layer sends a request for transmission to the PHYlayer. At the time, a parameter related to the information on the octetlength of the PSDU, L-LENGTH parameter, is determined based on atransmission time-related parameter, TXTIME parameter. TXTIME indicatesa transmission time determined for transmission of the PPDU includingthe PSDU by the PHY layer, corresponding to the transmission timerequested by the MAC layer for transmission of the PSDU (physicalservice data unit). Accordingly, the L-LENGTH parameter is atime-related parameter, and thus, the length subfield included in theL-SIG field 430 ends up containing transmission time-relatedinformation.

The VHT-SIGA field 440 includes control information (or signalinformation) necessary for the STAs receiving the PPDU to interpret thePPDU 400. The VHT-SIGA field 440 is transmitted in two OFDM symbols.Accordingly, the VHT-SIGA field 440 may be split into a VHT-SIGA1 fieldand a VHT-SIGA2 field. The VHT-SIGA1 field includes information on thechannel bandwidth used for PPDU transmission, identification informationrelated to whether STBC (Space Time Block Coding) is to be used,information indicating one of the SU or MU-MIMO scheme in which the PPDUis transmitted, information indicating a transmission target STA groupincluding a plurality of STAs MU-MIMO paired with the AP in case thetransmission scheme is MU-MIMO, and information on a spatial streamassigned to each STA included in the transmission target STA group. TheVHT-SIGA2 field includes short guard interval (GI)-related information.

The information indicating the MIMO transmission scheme and theinformation indicating the transmission target STA group may beimplemented as one piece of MIMO indication information, and as anexample, may be embodied as a group ID. The group ID may be set as avalue having a specific range, and in the range, a predetermined valueindicates the SU-MIMO transmission scheme, and the other values may beused as an identifier for the transmission target STA group in case thePPDU 400 is transmitted in the MU-MIMO transmission scheme.

If the group ID indicates that the PPDU 400 is transmitted through theSU-MIMO transmission scheme, the VHT-SIGA2 field includes codingindication information indicating whether the coding scheme applied tothe data field is BCC (Binary Convolution Coding) or LDPC (Low DensityParity Check) coding and MCS (modulation coding scheme) information on achannel between transmitter and receiver. Further, the VHT-SIGA2 fieldmay include a partial AID including the AID of the transmission targetSTA of the PPDU and/or some bit sequences of the AID.

If the group ID indicates that the PPDU 400 is transmitted through theMU-MIMO transmission scheme, the VHT-SIGA field 440 includes codingindicating information indicating whether the coding scheme applied tothe data field intended to be sent to the receiving STAs MU-MIMO pairedis BCC or LDPC coding. In such case, the MCS (modulation coding scheme)information on each receiving STA may be included in the VHT-SIGB field470.

The VHT-STF 450 is used for enhancing the ACG estimation capabilities inMIMO transmission.

The VHT-LTF 460 is used for an STA to estimate an MIMO channel. Sincethe next-generation wireless LAN system supports MU-MIMO, as manyVHT-LTFs 460 as the number of spatial streams where the PPDU 400 istransmitted may be configured. Additionally, full channel sounding issupported, and in case this is conducted, the number of VHT LTFs mayincrease.

The VHT-SIGB field 470 includes dedicated control information necessaryfor a plurality of MIMO paired STAs to receive the PPDU 400 to obtaindata. Accordingly, only when the control information included in thePPDU 400 indicates that the currently received PPDU 400 is MU-MIMOtransmitted, the STA may be designed to decode the VHT-SIGB field 470.On the contrary, in case the control information included in theVHT-SIGA field 440 indicates that the currently received PPDU 400 is onefor a single STA (including SU-MIMO), the STA may be designed not todecode the VHT-SIGB field 470.

The VHT-SIGB field 470 may contain information on the MCS (modulationand coding scheme) for each STA and information on rate matching.Further, it may contain information indicating the PSDU length includedin the data field for each STA. The information indicating the length ofthe PSDU is information indicating the length of the bit sequence of thePSDU and may perform such information on a per-octet basis. Meanwhile,in case the PPDU is SU-transmitted, the information on the MCS isincluded in the VHT-SIGA field 440, so that it might not be included inthe VHT-SIGB field 470. The size of the VHT-SIGB field 470 may varydepending on the type of the MIMO transmission (MU-MIMO or SU-MIMO) andchannel bandwidth used for transmission of the PPDU.

The data field 480 includes data which intends to be sent to the STA.The data field 480 includes a service field for initializing a scramblerand PSDU (PLCP Service Data Unit) where an MPDU (MAC Protocol Data Unit)is delivered in the MAC layer, a tail field including a bit sequencenecessary to turn the convolution encoder back into zero state, andpadding bits for normalizing the length of the data field. In the caseof MU transmission, the data field 480 transmitted to each STA mayinclude a data unit whose transmission is intended, and the data unitmay be an A-MPDU (aggregate MPDU).

In the wireless LAN system as shown in FIG. 1, in case the AP 10attempts to send data to STA1 21, STA2 22, and STA3 23, a PPDU may betransmitted to the STA group including STA1 21, STA2 22, STA3 223, andSTA4 24. In such case, as shown in FIG. 4, no spatial stream may beassigned to STA4 24, and a specific number of spatial streams areassigned to each of the STA1 21, STA2 22, and STA3 23, and data may betransmitted accordingly. In the example as illustrated in FIG. 4, onespatial stream may be assigned to STA1 21, three to STA2 22, and two toSTA3 23.

FIG. 5 is a block diagram illustrating the format of an MAC frameprovided in a wireless LAN system. The MAC frame may be an MPDU (in caseof being delivered in PHY layer, PSDU) included in the data field of theabove-described PPDU.

Referring to FIG. 5, the MAC frame 500 includes a frame control field510, a duration/ID field 520, an address 1 field 531, an address 2 field532, an address 3 field 533, a sequence control field 540, an address 4field 534, a QoS control field 550, an HT control field 560, a framebody 570, and an FCS (Frame Check Sequence) field 580.

The frame control field 510 includes information on framecharacteristics. The frame control field may contain protocol versioninformation indicating the version of the wireless LAN standardssupported by the frame 500 and information on the type and subtype foridentifying the function of the frame.

The duration/ID field 520 may be implemented to have different valuesdepending on the type and subtype of the MAC frame 500. In case the typeand subtype of the MAC frame 500 are PS-poll frames for power savingoperation, the duration/ID field 520 may be configured to include theAID of the STA that has sent the MAC frame 500. In other cases, theduration/ID field 520 may be configured to have a specific durationvalue depending on the type and subtype of the MAC frame 500. In casethe MAC frame 500 is an MPDU included in the A-MPDU format, theduration/ID field 520 included in the MAC header of each MPDU may beimplemented to have the same value.

The address 1 field 531 to the address 4 field 534 may be configured toimplement specific fields among a BSSID field indicating a BSSID, an SAfield indicating a source address (SA), a DA field indicating adestination address (DA), a TA (transmitting address) field indicating atransmitting STA address, and an RA (Receiving Address) field indicatinga receiving STA address. Meanwhile, the address field embodied as a TAfield may be set as a bandwidth signaled TA value, and in such case, theTA field may indicate that the frame contains additional information inthe scrambling sequence. The bandwidth signaled TA may be represented inan MAC address of the STA transmitting the frame, but theindividual/group bit included in the MAC address may be set as apredetermined value, e.g., 1.

The sequence control field 540 is configured to include a sequencenumber and a fragment number. The sequence number may indicate asequence number assigned to the MAC frame 500. The fragment number mayindicate the number of each fragment in the MAC frame 500.

The QoS control field 550 includes information related to QoS.

The HT control field 560 includes control information related to a highthroughput (HT) transmission/reception scheme and/or very highthroughput (VHT) transmission/reception scheme. The implementation ofthe HT control field 560 is described in further detail below.

The frame body 570 may include data that a receiving STA and/or APintends to send. The frame body 570 may include a control frame, amanagement frame, an action frame, and/or a data frame with a bodycomponent except the MAC header and FCS. In case the MAC frame 500 is amanagement frame and/or action frame, the information elements containedin the management frame and/or action frame may be implemented in theframe body 570.

The FCS field 580 includes a bit sequence for CRC.

Hereinafter, the above-described HT control field is described ingreater detail with some drawings.

FIG. 6 is a block diagram illustrating the format of an HT controlfield.

Referring to FIG. 6, the HT control field 560 includes a VHT variantfield 561, an HT control middle field 562, an AC constraint field 563,and a RDG/More PPDU field 564.

The VHT variant field 561 indicates whether the HT control field 560 hasan HT control field format for VHT or HT control field format for HT. Asan example, the VHT variant field 561 may be embodied as a field havinga one-bit length, and depending on the value, it may be indicatedwhether the HT control middle field 562 is realized to have a format forHT or format for VHT.

The HT control middle field 562 may be implemented to have a differentformat depending on the indication of the VHT variant field 561. Thespecific implementation of the HT control middle field 562 is describedin further detail below.

The AC constraint field 563 indicates whether the mapped AC (AccessCategory) of an RD (Reverse Direction) data frame is restricted to asingle AC.

The RDG/More PPDU field 564 may be interpreted in different waysdepending on whether the field is transmitted by an RD initiator or RDresponder. When transmitted by the RD initiator, if the RDG/More PPDUfield is set as ‘1’, it can be interpreted that there is an RDG and thismay be defined by the duration/ID field. When sent by the RD responder,if the RDG/More PPDU field is set as ‘0,’ it can be interpreted that thePPDU including the same indicates the last frame as transmitted by theRD responder. If the RDG/More PPDU field is set as 1, it can beconstrued that subsequent to the PPDU including the same, other PPDU isto be transmitted.

FIG. 7 is a block diagram illustrating the format of an HT variantmiddle field for HT.

Referring to FIG. 7, the HT variant middle field 700 for HT includes alink adaptation control subfield 710, a calibration position subfield720, a calibration sequence subfield 730, a CSI (Channel StateInformation)/steering subfield 740, and an NDP (Null Data Packet)announcement subfield 750.

The link adaptation control subfield 710 may include a TRQ (trainingrequest) subfield 711, an MAI (MCS request or ASEL (antenna selection)Indication) subfield 712, an MFSI (MCAS feedback sequence identifier)subfield 713 and an MFB/ASELC (MCS feedback and ASEL command/data)subfield 714.

The TRQ subfield 711 includes information for requesting that a soundingresponder send a sounding frame. The MAI subfield 712 may containindication information for requesting MCS feedback or informationindicating that the MFB/ASELC subfield 714 contains antenna selectionindication information. The MAI subfield 712 includes an MCS request(MRQ) indication bit and may contain an MSI (MRQ Sequence Identifier)subfield having a sequence number that enables identification of theMRQ. By setting the value of the subfield, whether MCS feedback isrequested may be denoted. The MFSI subfield 713 may be set as a receivedvalue of the MSI included in the MFB information-related frame. TheMFB/ASELC subfield 714 contains MFB information or antenna selectionindication information.

The calibration position subfield 720 and the calibration sequencesubfield 730 include the position of a calibration sounding exchangesequence and identification information of a calibration sequence.

The CSI/steering subfield 740 indicates information denoting a feedbacktype.

The NDP announcement subfield 750 may be set as NDP announcementindication information indicating that an NDP is to be sent subsequentto the PPDU currently transmitted. The NDP announcement subfield 750 maybe configured to have a one-bit size, and when receiving a PPDU, the STAmay verify whether the PPDU is an NDPA frame through the value of theNDP announcement subfield 750.

FIG. 8 is a block diagram illustrating the format of an HT variantmiddle field for VHT.

Referring to FIG. 8, the HT variant middle field 800 for VHT includes anMRQ subfield 810, an MSI subfield 820, an MFSI/GID-L subfield 830, anMFB subfield 840, a GID-H subfield 850, a coding type subfield 860, anFB Tx type subfield 870, and an unsolicited MFB subfield 880.

The MRQ subfield 810 indicates whether to request MCS feedback. If theMRQ subfield 810 is set as 1, it can be implemented that MCS feedback isrequested.

The MSI subfield 820 includes, when the MRQ subfield 810 indicatesrequesting MCS feedback, a sequence number for identifying the specificrequest.

The unsolicited MFB subfield 880 may indicate whether the included MFBinformation responds to the MRQ. If the unsolicited MFB subfield 880 isset as 1, the included MFB information may be implemented to be aresponse to the MRQ. If the unsolicited MFB subfield 880 is set as 0,the included MFB information may be implemented to be not a response tothe MRQ.

The MFSI/GID-L subfield 830 may be construed in different ways dependingon the configuration of the unsolicited MFB subfield 880. If theunsolicited MFB subfield 880 indicates that the included MFB informationis a response to the MRQ, it may include the reception value of the MSIcontained in the MFB information-related frame. If the unsolicited MFBsubfield 880 indicates that the included MFB information is not aresponse to the MRQ, it may contain lowest three bits constituting thegroup ID of the PPDU related to the unsolicited MFB information.

The MFB subfield 840 may include recommended MFB information. The MFBsubfield 840 may include a VHT N_STS subfield 841, an MCS subfield 842,a BW subfield 843, and an SNR subfield 844. The VHT N_STS subfield 841indicates the number of recommended spatial streams. The MCS subfield842 indicates a recommended MCS (modulation coding scheme). The BWsubfield 843 indicates bandwidth information related to the recommendedMCS. The SNR subfield indicates an average SNR value over a spatialstream and a data subcarrier.

The GID-H subfield 850 may include highest three bits constituting agroup ID of a PPDU related to the unsolicited MFB information if theunsolicited subfield 880 indicates that the MFB information is not aresponse to the MRQ and the MFB is estimated from a PPDU for MUtransmission and reception. If the MFB is estimated from a PPDU for SUtransmission and reception, the GID-H subfield 850 may include a bitsequence set as 1.

The coding type subfield 860, in case the unsolicited MFB subfield 880indicates that the MFB information is not a response to the MRQ, mayinclude coding information (BCC or LDPC) of the frame where theunsolicited MFB information has been estimated.

The FB Tx type subfield 870 may be configured to indicate thetransmission type of the estimated PPDU. That is, it may indicatewhether the estimated PPDU has been beamformed.

Whether the VHT variant field 561 is separated into the HT control fieldfor VHT and HT control field for HT may be done based on the controlinformation included in the HT control middle field 562.

Meanwhile, the next-generation wireless LAN system supports MU-MIMO(multi user multiple input multiple output) scheme in which a pluralityof STAs simultaneously gain access to the channel in order toefficiently use the radio channel According to the MU-MIMO transmissionscheme, the AP may transmit packets to one or more MIMO-paired STAs atthe same time.

Always sensing a channel for frame transmission and reception causes theSTA to continue to consume power. The power consumption in the receptionstate makes little difference as compared with the power consumption inthe transmission state, so that keeping the reception state causes theSTA battery powered to consume relatively more power. Accordingly, whenin the wireless LAN system an STA conducts channel sensing whilecontinuously maintaining the reception waiting state, inefficient powerconsumption may arise without particularly increasing wireless LANthroughput, and thus, it is inappropriate in view of power management.

To compensate for such problems, the wireless LAN system supports apower management (PM) mode for an STA. The STA power management mode isseparated into an active mode and a power save (PS) mode. The STAoperates basically in the active mode. The STA operating in the activemode maintains an awake state. That is, the STA remains at a state ofbeing able to perform normal operation such as frame transmission andreception or channel sensing.

When in normal operation, the STA shifts between the doze state andawake state. In the doze state, the STA operating with the minimum powerand does not receive radio signals including data frames from the AP.Further, in the doze state, the STA does not conduct channel sensing.

As the STA operates as long as possible, power consumption decreases, sothat the operation period of the STA is increased. However, since frametransmission and reception is impossible in the doze state, it cannot beleft at the operation state unconditionally. In case there is a frame tobe transmitted from the STA operating in the doze to the AP, the STAshifts to the awake state, thereby able to receive frames. However, incase the AP has a frame to be transmitted to the STA operating in thedoze state, the STA cannot receive the frame nor is the STA able to beaware of the existence of the STA. Accordingly, the STA may require theoperations of being aware of whether there is a frame to be sent to theSTA, and if any, shifting to the awake state at a specific period so asto receive the frame. This is described below in connection with FIG. 9.

FIG. 9 is a view illustrating an example of power management operation.

Referring to FIG. 9, the AP 910 sends a beacon frame to STAs in a BSS ata constant period (S910). The beacon frame includes a TIM (trafficindication map) information element. The TIM element includesinformation indicating that the AP 910 buffers a bufferable frame (orbufferable unit; BU) for the STAs associated with the AP 910 and thatthe frame is to be sent. The TIM element includes a TIM used to indicatea unicast frame and a DTIM (delivery traffic indication map) used toindicate a multicast or broadcast frame.

The AP 910 transmits a DTIM once every three beacon frames oftransmission.

STA1 921 and STA2 922 are STAs operating in PS mode. STA1 921 and STA2922 shift from the doze state to the awake state at every wakeupinterval of a specific period so that the STAs may receive the TIMelement transmitted from the AP 910.

A specific wakeup interval may be configured so that STA1 921 may shiftto the awake state at every beacon interval to thus receive a TIMelement. Accordingly, when the AP 910 first sends out a beacon frame(S911), STA1 921 switches to the awake state (S921). STA1 921 receivesthe beacon frame and obtains the TIM element. In case the obtained TIMelement indicates that a bufferable frame to be sent to STA1 921 isbeing buffered, STA1 921 transmits a PS-poll frame to the AP 910 torequest that the AP 910 send a frame (S921a). In response to the PS-pollframe, the AP 910 sends a frame to STA1 921 (S931). When completelyreceiving the frame, STA1 921 turns back to the doze state.

When the AP 910 sends out a second beacon frame, since the medium isoccupied, for example, as if another device gains access to the medium,the AP 910 fails to send a beacon frame at exact beacon interval and maydeferred transmission of the beacon frame (S912). In such case, STA1 921turns its operation mode to the awake state according to the beaconinterval, but cannot receive the deferred beacon frame, so that STA1 921switches back to the doze state (S922).

When the AP 910 sends out a third beacon frame, the beacon frame mayinclude a TIM element that is set as DTIM. However, since the medium isoccupied, the AP 910's transmission of the beacon frame is deferred(S913). STA1 921 switches to the awake state in accordance with thebeacon interval and may obtain the DTIM through the beacon frametransmitted by the AP 910. The DTIM obtained by STA1 921 indicates thatthere is no frame to be transmitted to STA1 921 and that there is aframe for other STA. Accordingly, STA1 921 shifts back to the dozestate. The AP 910, after transmission of the beacon frame, sends a frameto the STA (S932).

The AP 910 sends a fourth beacon frame (S914). However, STA1 921 couldnot obtain the information indicating that a bufferable frame for itselfremains buffered through the previous twice reception of the TIMelement, and thus, STA1 921 may adjust the wakeup interval for receptionof a TIM element. Or, in case the beacon frame transmitted by the AP 910includes signaling information for adjusting the wakeup interval valueof STA1 921, the wakeup interval value of STA1 921 may be adjusted. Inthis example, STA1 921 may change its configuration so that shift of theoperation state for receiving a TIM element is performed at every threebeacon intervals rather than at every beacon interval. Accordingly, STA1921 stays at the doze state after the AP 910 sends a fourth beacon frame(S914) and when the AP 910 transmits a fifth beacon frame (S915), andthus, it cannot obtain the TIM element.

When the AP 910 sends out a sixth beacon frame (S916), STA1 921 switchesto the awake state and obtains the TIM element included in the beaconframe (S924). The TIM element is a DTIM indicating that there is abroadcast frame, so that STA1 921 does not transmit a PS-poll frame tothe AP 910 and receives a broadcast frame transmitted by the AP 910(S934).

Meanwhile, the wakeup interval configured in STA2 922 may have a longerperiod than that of STA1 921. Accordingly, when the AP 910 sends a fifthbeacon frame (S915), STA2 922 may switch to the awake state to receive aTIM element (S925). STA2 922 is aware that there is a frame to be sentthereto through the TIM element, and in order to request transmission,sends a PS-poll frame to the AP 910 (S925 a). The AP 910 sends a frameto STA2 922 in response to the PS-poll frame (S933).

In order to operate the power save mode as shown in FIG. 9, the TIMelement includes a TIM indicating whether there is a frame to be sent tothe STA or a DTIM indicating whether there is a broadcast/multicastframe. The DTIM may be embodied by configuring a field of the TIMelement.

FIG. 10 is a block diagram illustrating an example of a TIM elementformat.

Referring to FIG. 10, the TIM element 1000 includes an element ID field1010, a length field 1020, a DTIM count field 1030, a DTIM period field1040, a bitmap control field 1050, and a partial virtual bitmap field1060.

The element ID field 1010 indicates that an information element is a TIMelement. The length field 1020 indicates the whole length includingitself and subsequent fields. The maximum value may be 255 and may beset in octets.

The DTIM count field 1030 indicates whether a current TIM element is aDTIM, and unless it is a DTIM, indicates the number of remaining TIMsuntil the DTIM is transmitted. The DTIM period field 1040 indicates aperiod at which the DTIM is transmitted, and the period at which theDTIM is transmitted may be set as a multiple of the count oftransmission of a beacon frame.

The bitmap control field 1050 and the partial virtual bitmap field 1060indicate whether a specific STA buffers a bufferable frame. The firstbit in the bitmap control field 1050 indicates whether there is amulticast/broadcast frame to be sent. The remaining bits are set toindicate an offset value to interpret the subsequent partial virtualbitmap field 1060.

The partial virtual bitmap field 1060 is set as a value indicatingwhether there is a bufferable frame to be sent to each STA. This may beset in the bitmap form where a bitmap corresponding to the AID value ofa specific STA is set as 1. According to the AID order, allocation maybe done from 1 to 2007, and as an example, if the fourth bit is set as1, it means that traffic is buffered in the AP which is to be sent tothe STA whose AID is4.

Meanwhile, in the circumstance where bits set as consecutive 0's come upfrequently in configuring the bit sequence of the partial virtual bitmapfield 360, using the whole bit sequence configuring the bitmap may beinsufficient. For this, the bitmap control field 1050 may contain offsetinformation for the partial virtual bitmap field 1060.

FIG. 11 is a view illustrating an example of a bitmap control field anda partial virtual bitmap field according to an embodiment of the presentinvention.

Referring to FIG. 11, the bitmap sequence constituting the partialvirtual bitmap field 1060 indicates whether the STA having an AIDcorresponding to the bitmap index includes a buffered frame. The bitmapsequence constitutes indication information on AIDs 0 to 2007.

The bitmap sequence may have consecutive 0's from the first bit to thekth bit. Further, consecutive 0's may be set from the other lth bit tothe last bit. This indicates that the STAs assigned AIDs 0 to k and theSTAs assigned with 1 to 2007 do not have any buffered frame. As such,the sequence of consecutive 0's from 0th to the kth in the early part ofthe bitmap sequence may be provided offset information and the sequenceof 0's in the latter part may be omitted, thereby reducing the size ofthe TIM element.

For this, the bitmap control field 1050 may include a bitmap offsetsubfield 1051 that contains offset information of a sequence ofconsecutive 0's in the bitmap sequence. The bitmap offset subfield 1051may be set to indicate k, and the partial virtual bitmap field 1060 maybe set to include the k+1th bit to the 1−1th bit of the original bitmapsequence.

A detailed responding procedure of the STA that has received the TIMelement is described with reference to FIGS. 12 to 14.

FIG. 12 is a flowchart illustrating an example of an AP's respondingprocedure in a TIM protocol.

Referring to FIG. 12, the STA 1220 shifts its operation state from dozestate to awake state in order to receive a beacon frame including a TIMfrom the AP 1210 (S1210). The STA 1220 may be aware that there is abuffered frame to be sent thereto by interpreting the received TIMelement.

The STA 1220 contends with other STAs for medium access to transmit aPS-poll frame (S1220) and sends a PS-poll frame to the AP 1210 forrequesting transmission of a data frame (S1230).

When receiving the PS-poll frame transmitted from the STA 1220, the AP1210 sends a frame to the STA 1220 (S1240). The STA 1220 receives thedata frame and in response transmits an ACK (acknowledgement) frame tothe AP 1210 (S1250). Thereafter, the STA 1220 shifts its operating modeback into the doze state (S1260).

As shown in FIG. 12, the AP may transmit data at a specific time afterreceiving the PS-poll frame rather than sending a data frame right afterreceiving the PS-poll frame from the STA.

FIG. 13 is a flowchart illustrating another example of an AP'sresponding procedure in a TIM protocol.

Referring to FIG. 13, the STA 1320 shifts its operation mode from dozestate to awake state in order to receive a beacon frame including a TIMfrom the AP 1310 (S1310). The STA 1320 may be aware that there is abuffered frame to be sent thereto by interpreting the received TIMelement.

The STA 1320 contends with other STAs for medium access for transmissionof the PS-poll frame (S1320) and sends the PS-poll frame to the AP 1310for requesting the transmission of a data frame (S1330).

In case, despite receiving the PS-poll frame, the AP 1310 fails toprepare for a data frame for a specific time interval, the AP 1310,instead of immediately transmitting a data frame, sends an ACK frame tothe STA 1320 (S1340). This is a feature of a deferred response differentfrom step S1240 in which the AP 1210 shown in FIG. 12 sends a data frameto the STA 1220 immediately in response to the PS-poll frame.

The AP 1310, if a data frame is ready after transmission of the ACKframe, performs contention (S1350), and then sends a data frame to theSTA 1320 (S1360).

The STA 1320 sends an ACK frame to the AP 1310 in response to receptionof the data frame (S1370) and switches its operation mode to the dozestate (S1380).

If the AP sends a DTIM to the STA, a TIM protocol procedure that isperformed thereafter may differ.

FIG. 14 is a flowchart illustrating a procedure of a TIM protocol by aDTIM.

Referring to FIG. 14, STAs 1420 switch their operation mode from thedoze state to the awake state in order to receive a beacon frameincluding a TIM element (S1410). The STAs 1420 may be aware that amulticast/broadcast frame is to be transmitted through the receivedDTIM.

The AP 1410 sends out a multicast/broadcast frame after transmission ofthe beacon frame including the DTIM (S1420). The STAs 1420 switch theiroperation state back to the doze state after receiving themulticast/broadcast frame transmitted by the AP 1410.

In the power save mode operation method based on the TIM protocoldescribed in connection with FIGS. 9 to 14, the STAs may verify whetherthere is a buffered frame to be transmitted due to buffered trafficthrough the STA identification information included in the TIM element.The STA identification information may be information associated with anAID (Association Identifier) that is an identifier assigned when the STAis associated with the AP. The STA identification information may beconfigured to directly indicate the AIDs of the STAs having a bufferedframe or may be configured in the bitmap type in which a bit ordercorresponding to the AID value is set as a specific value. The STAs maybe aware that there is a frame buffered thereto if the STAidentification information indicates its AID.

Meanwhile, a power management operation based on APSD (Automatic PowerSave Delivery) may also be offered in order for saving power of astation.

The AP that may support APSD signals that the APSD may be supportedthrough use of the APSD subfield included in the capabilitiesinformation field of the association response frame, probe responseframe, and beacon frame. The STA that may support APSD uses a powermanagement field that is included in the frame control field of theframe in order to indicate whether it operates in the active mode orpower save mode.

The APSD is a mechanism for delivering downlink data and a bufferablemanagement frame to an STA that is operating in the power save mode. Inthe frame that is transmitted by the STA that stays in the power savemode and is using APSD, the power management bit of the frame controlfield is set as 1, and through this, buffering may arise in the AP.

The APSD defines two delivery mechanisms such as U-APSD(Unscheduled-APSD) and S-APSD (Scheduled-APSD). The STA may use theU-APSD so that part or whole of its BU (Bufferable Unit) is deliveredduring an unscheduled SP (Service Period). The STA may use the S-APSD sothat part or whole of its BU is delivered during a scheduled SP.

The STA using the U-APSD might not receive a frame transmitted by the APduring a service period due to interference. Although the AP might notsense interference, the AP may determine that the STA failed to exactlyreceive the frame. The U-APSD co-existence capability value enables theSTA to inform requested transmission duration to the AP so that it canbe used as a service period for the U-APSD. The AP may transmit a frameduring the service period, and accordingly, may enhance the possibilityof being able to receive a frame while the STA is under interference.Further, the U-APSD may reduce possibility of failing to receive a frametransmitted from the AP during the service period.

The STA transmits to the AP an ADDTS (Add Traffic Stream) request frameincluding a U-APSD coexistence element. The U-APSD coexistence elementmay include information on the requested service period.

The AP treats the requested service period, and in response to the ADDTSrequest frame, may send an ADDTS response frame. The ADDTS request framemay include a state code. The state code may indicate responseinformation on the requested service period. The state code may indicatewhether the requested service period is allowed, and in case therequested service period is refused, may further indicate a reason forthe refusal.

In case the requested service period is allowed by the AP, the AP maysend a frame to the STA during the service period. The duration of theservice period may be specified by the U-APSD coexistence elementincluded in the ADDTS request frame. The start of the service period maybe a time when the AP normally receives a trigger frame transmitted fromthe STA.

The STA may enter into the doze state if the U-APSD service periodexpires.

Meanwhile, as various communication services such as smart grid ande-Health, or ubiquitous services appear, the M2M (Machine to Machine) tosupport such services draws attention. A sensor for sensing temperatureor moisture, a camera, a home appliance such as a TV, or a bulky machineincluding a factory processing machine or a vehicle may be one elementof an M2M system. Elements constituting an M2M system may transmit andreceive data based on WLAN communication. In case devices of an M2Msystem supports WLAN and configure a network, the system is hereinafterreferred to as an M2M wireless LAN system.

An M2M-supportive wireless LAN system may make use of a frequency bandof 1GHz or more, and use of a low band frequency may cause the servicecoverage to be expanded. Accordingly, the number of wireless deviceslocated in the service coverage may be larger than the number ofwireless devices in the existing wireless LAN system. Further, theM2M-supportive wireless LAN system has the following features.

1) many STAs: M2M assumes that unlike the existing networks a greatnumber of STAs are present in a BSS. This is because sensors installedin the home or business as well as devices owned by individuals areconsidered. Accordingly, a considerable number of STAs may be linked toone AP.

2) Low traffic load per STA: Since an M2M terminal has a traffic patternof gathering ambient information and reporting, it need not be sentfrequently and the amount of information is relatively small.

3) Uplink-centered communication: M2M has the structure of receiving acommand mainly on downlink, taking action, and reporting resultant dataon uplink. The main data is generally sent on uplink, and thus, anM2M-supportive system becomes uplink centered.

4) STA's power management: An M2M terminal is primarily battery powered,and in many cases, it is difficult to recharge often. Accordingly, apower management method is required to minimize battery consumption.

5) Automatic restoration function: A device constituting an M2M systemis difficult for a human being to manipulate in a specific circumstance,and thus, the device requires a self-restoration function.

In accordance with a server/client structure in a general wireless LANsystem, a client such as STA sends a request for information to aserver, and the server sends information to the STA in response to therequest. At the time, the server that has provided information may beconsidered a machine that mechanically collects and offers information,and the party that has received the information may be a user using theclient. Due to such structural nature, downlink-oriented communicationtechnology has been mainly developed in the existing wireless LANsystems.

On the contrary, in the M2M-supportive wireless LAN system, an oppositeof the above structure applies. In other words, the client, a machine,gathers and provides information, and the user managing the server mayrequest information. That is, in the M2M-supportive wireless LAN system,the M2M server issues a command related to ambient environmentmeasurement to M2M STAs and the M2M STAs conduct operation per thecommand and report the collected information to the server, in generalcommunication flow. Unlike the previous, the user happens to access thenetwork in the side of the server, and the communication flow goes inthe opposite direction. These are structural features of theM2M-supportive wireless LAN system.

In the above wireless LAN environment, a power save mechanism may beoffered which prevents the STA from unnecessarily maintaining the awakestate, and if identified that there is a buffered frame, enables the STAto switch to the awake state in order to receive the buffered frame.

The STA transmitting and receiving a frame based on the power savemechanism may be conducted based on the TIM protocol as shown in FIGS. 9to 14. According to the TIM protocol, the AP sends a data frame afterreceiving a PS-poll frame from the STA, and in this case, the AP maytransmit one buffered frame, i.e., a PSDU, in response to the PS-pollframe. Meanwhile, the AP transmitting only one buffered frame inresponse to the PS-poll frame in the environment where there is highbuffered traffic for the STA is not efficient in view of traffictreatment.

To address the above problems, a U-APSD may apply to a method oftransmitting and receiving a frame based on a TIM protocol. The STA mayreceive at least one or more frames from the AP during a service periodfor itself.

FIG. 15 is a view illustrating an example method of transmitting andreceiving a frame based on a TIM protocol and U-APSD.

Referring to FIG. 15, the STA, which stays in the doze state, entersinto the awake state in order to receive a TIM element (S1511).

The STA receives a TIM element (S1512). The TIM element may betransmitted, included in a beacon frame. When receiving the TIM element,the terminal may determine whether a bufferable frame for itself isbeing buffered based on the AID of the STA and the bitmap sequence ofthe partial virtual bitmap field included in the TIM element.

When identifying that there is a buffered frame, the STA enters backinto the doze state (S1513).

At a time when the buffered frame desires to be transmitted, the STAswitches back to the awake state and obtains a channel access authoritythrough contention (S1521). The STA acquires the channel accessauthority and transmits a trigger frame to thereby notify that a serviceperiod for the STA has been initiated (S1522).

The AP sends an ACK frame to the STA in response to the trigger frame(S1523).

The AP may conduct an RTS/CTS exchange procedure to transmit a bufferedframe within a service period. The AP obtains a channel access authoritythrough contention in order to send an RTS frame (S1531). The APtransmits an RTS frame to the STA (S1532), and the STA sends a CTS frameto the AP in response thereto (S1533).

The AP transmits a data frame related to at least a buffered frame afterRTS/CTS exchange at least once or more (S1541, S1542, and S1543). If theAP conducts the last transmission of a frame with (EOSP) in the QoSservice field of the frame set as ‘1,’ the STA may then receive the lastframe and may recognize that the service period is to be terminated.

The STA sends an ACK frame to the AP in response to the, at least, oneframe received when the service period is terminated (S1550). At thetime, the ACK frame may be a block ACK, an acknowledgement of receptionfor a plurality of frames. The STA that has transmitted the ACK frameenters into the doze state (S1560).

By the frame transmission/reception method described above in connectionwith FIG. 15, the STA may begin the service period at a desired time andmay receive at least one or more frames during one service period.Accordingly, efficiency may be enhanced in light of traffic processing.

Meanwhile, in the above-described frame transmission/reception method,the RTS/CTS frame exchange required upon transmission of data in orderto prevent a hidden node problem puts heavy overhead on datatransmission. Further, in the U-APSD, it takes a while after the STAsends a trigger frame to request that the AP send data and the AP thenprepares for data to be sent and subsequently conducts contention fordata transmission. Since the STA may happen to unnecessarily maintainthe awake state for the time, the power save efficiency may be lowered.

Accordingly, the present invention suggests a method of being able tomore efficiently transmit a data frame that is prepared for transmissionto the STA by the AP in advance starting a service period at a scheduledtime between the AP and the STA when the STA receives data from the AP.

For this the present invention suggests an SP (Service Period)-pollframe.

FIG. 16 is a block diagram illustrating an MAC frame format of anSP-poll frame according to an embodiment of the present invention.

Referring to FIG. 16, the SP-poll frame 1600 may include a frame controlfield 1610, a duration field 1620, a BSSID(RA) field 1630, a TA field1640, a frame body 1650, and an FCS field 1660.

The frame control field 1610 may indicate that the frame is an SP-pollframe.

The duration field 1620 may indicate duration of a polled service periodinitiated by the SP-poll frame 1600. The duration field 1620 may be abasis for configuring an NAV (Network Allocation Vector) of other STAthat does not send the SP-poll frame 1600.

The BSSID(RA) field 1630 may include identification information of theAP or identification information of the BSS operated by the APassociated with the STA. The identification information may be a BSSID.

The TA field 1640 may include identification information of the STA thathas transmitted the SP-poll frame 1600. The identification informationmay be the MAC address of the STA. The identification information mayinclude an AID of the STA.

The frame body 1650 may include a polled service period interval (polledSP interval) field. The polled service period field may includeinformation related to the polled SP interval that is an intervalbetween when the service period initiated by the SP-poll frame 1600expires and when a subsequent service period is initiated. The polled SPfield may include information related to a time when the SP-poll frame1600 is transmitted and then a next SP-poll frame is transmitted.

The FCS field 1660 may include a sequence for CRC.

The polled SP field indicating the interval between service periodsand/or interval in the transmission between SP-poll frames may be set toindicate that the interval value is 0 and/or Null. This may be toindicate that the polled service period is initiated by the SP-pollframe transmitted by the STA and at least one or more frames are to betransmitted from the AP within the service period. Further, the fieldthusly set may be to indicate not considering that after the polledservice period initiated by the SP-poll frame, a polled service periodis initiated again to transmit and receive a buffered frame.

The frame transmission/reception method by the power save mode STA basedon the above-described SP-poll frame may be split into an immediateSP-poll mechanism and a deferred SP-poll mechanism depending on theresponse of the AP that has received the SP-poll frame.

FIG. 17 is a view illustrating an example method of transmitting andreceiving a frame by an STA operating in a power save mode according toanother embodiment of the present invention. The frame transmission andreception method shown in FIG. 17 may be an example of a frametransmission and reception method according to the immediate SP-pollmechanism.

Referring to FIG. 17, the STA that stays in the doze state enters intothe awake state in order to receive a TIM element (S1710).

The STA receives the TIM element (S1720). The TIM element may betransmitted, included in a beacon frame. When receiving the TIM element,the terminal may determine whether a bufferable frame for itself isbeing buffered based on the AID of the STA and the bitmap sequence ofthe partial virtual bitmap field included in the TIM element.

When identifying that a bufferable frame is being buffered, the STAobtains a channel access authority through contention and may send arequest for transmission of a buffered frame to the AP throughtransmission of an SP-poll frame (S1730).

When receiving the SP-poll frame, the AP sends at least one or morebuffered frames to the STA after the SIFS (S1741, S1742, and S1743). Insuch case, the AP may continuously transmit a plurality of bufferedframes during a polled service period.

In case a specific polled service period is not configured throughseparate signaling between the AP and the STA, the EOSP value may be setas 1 in the last buffered frame that is transmitted from the AP to theSTA during the polled service period. Through this, the polled serviceperiod between the STA and the AP may expire.

On the contrary, a specific polled service period may be configuredthrough separate signaling between the AP and the STA. For this, theduration field of the SP-poll frame transmitted from the STA may apply.In such case, the polled service period may be initiated at a time whenthe STA sends the SP-poll frame or when the AP receives the SP-pollframe. The polled service period may be configured during the timeperiod indicated by the duration field from the time of initiation. TheAP may send a buffered frame in compliance with the duration of thepolled service period. The STA may receive a buffered frame inaccordance with the duration of the polled service period.

The STA may send an ACK frame to the AP (S1750). The STA enters into thedoze state after transmitting the ACK frame (S1760). The ACK frame maybe sent at a time when the polled service period expires.

FIG. 18 is a view illustrating another example of a method oftransmitting and receiving a frame by an STA operating in a power savemode according to another embodiment of the present invention. The frametransmission and reception method shown in FIG. 18 is based on thedeferred SP-poll mechanism.

Referring to FIG. 18, the STA that stays in the doze state enters intothe awake state in order to receive a TIM element (S1811).

The STA receives the TIM element (S1812). The TIM element may be sentincluded in a beacon frame. When receiving the TIM element, the terminalmay determine whether a bufferable frame for itself is being bufferedbased on the AID of the STA and the bitmap sequence of the partialvirtual bitmap field included in the TIM element.

When identifying that a bufferable frame is being buffered, the STAobtains a channel access authority through contention (S1821) and maysend a request for transmission of a buffered frame to the AP throughtransmission of an SP-poll frame (S1822). As the SP-poll frame istransmitted, a first polled service period may be initiated.

Meanwhile, the AP, after receiving the SP-poll frame, might not send abuffered frame to the STA within the SIFS. In such case, the APtransmits an ACK frame after receiving the SP-poll frame (S1823).

When receiving the ACK frame in response to the transmitted SP-pollframe, the STA may recognize that the AP cannot send a buffered frame.In this case, the first polled service period that has been initiated bytransmission of the SP-poll frame may expire. The STA receives an ACKframe and enters into the doze state (S1824).

Meanwhile, the STA enters into the awake state at a time indicated bythe polled SP interval field of the SP-poll frame (S1831) and obtains achannel access authority through contention (S1832).

When obtaining the channel access authority, the STA requests that theAP send a frame through transmission of the SP-poll frame (S1833). Asecond polled service period begins through the transmission of theSP-poll frame.

Meanwhile, the AP may previously grasp the time when the STA intends toinitiate the second polled service period through the polled SP intervalfield of the received SP-poll frame. As an example, in case the polledSP interval field indicates an interval between two polled serviceperiods, it can be possible to know the time when the STA intends toinitiate the second polled service period and receives a buffered frameby interpreting the polled SP interval field of the SP-poll frame instep S1822. As another example, in case the polled SP interval fieldindicates an interval when the STA that has already transmitted anSP-poll frame intends to send a subsequent SP-poll frame, the AP mayknow the time that the STA intends to send an SP-poll frame byconstruing the polled SP interval field included in the SP-poll frame instep S1822. However, FIG. 18 illustrates a polled SP interval when it isassumed that the polled SP interval field indicates the time when a nextSP-poll frame is intended to be sent.

Accordingly, the AP may prepare for a buffered frame to be transmittedto the STA an SIFS after receiving the SP-poll frame. The AP maytransmit one or more buffered frames to the STA during the second polledservice period the SIFS after receiving the SP-poll frame (S1841, S1842,and S1843).

The duration of the second polled service period initiated by the STAtransmitting the SP-poll frame (S1833) may be specified as the durationof the polled period described above in connection with FIG. 17. Thatis, the polled duration may be terminated by the AP sending out abuffered frame including an EOSP field set as 1. Or, the second polledduration may be specified by the duration indicated by the durationfield of the SP-poll frame transmitted by the STA in step S1833.

The STA receives at least one or more frames from the AP, and inresponse, transmits an ACK frame (S1844). The ACK frame transmitted bythe STA may be a block ACK as a response to acknowledge reception of atleast one or more buffered frames. The STA may enter into the doze stateafter transmitting the ACK frame (S1850).

Although in FIG. 18 the second polled service period expires after theSTA sends the ACK frame, the second polled service period may beterminated right before the STA transmits the ACK frame. That is, theSTA may be configured to transmit an ACK frame to the AP when the secondpolled service period ends.

In the frame transmission and reception method shown in FIG. 18, the APduring the first polled service period transmits an ACK frame inresponse to the STA's SP-poll frame. Accordingly, during the firstpolled service period, a deferred SP-poll-based frame transmission andreception method is carried out. During the second polled serviceperiod, the AP sends at least one buffered frame in response to theSTA's SP-poll frame. Accordingly, an immediate SP-poll-based frametransmission and reception method is conducted during the second polledservice period.

Meanwhile, since the period during which one STA can occupy a channel islimited, there is a limit to the amount of data that may be sent fromthe AP to the STA during one polled service period. Accordingly, heavytraffic buffered for the STA may render it difficult to process all thebuffered frames by sending the buffered frames during one polled serviceperiod. In such case, the buffered traffic may be treated throughresuming the polled service period. The time of the start of asubsequent polled service period may be signaled by the polled SPinterval field of the SP-poll frame that triggers an immediately priorpolled service period. This is described below in greater detail withreference to drawings.

FIG. 19 is a view illustrating another example of a method oftransmitting and receiving a frame by an STA operating in a power savemode according to an embodiment of the present invention.

Referring to FIG. 19, the STA switches from the doze state to the awakestate so as to receive a TIM element (S1911).

The STA receives the TIM element (S1912). The TIM element may betransmitted included in a beacon frame. When receiving the TIM element,the terminal may determine whether a bufferable frame for itself isbeing buffered based on the AID of the STA and the bitmap sequence ofthe partial virtual bitmap field included in the TIM element.

When identifying that a bufferable frame is being buffered, the STAobtains a channel access authority through contention (S1921) and maysend a request for transmission of a buffered frame to the AP throughthe SP-poll frame (S1922). By the transmission of the SP-poll frame, afirst polled service period may be initiated.

Meanwhile, the AP might not send a buffered frame to the STA an SIFSafter receiving the SP-poll frame. In this case, the AP sends an ACKframe after receiving the SP-poll frame (S1923).

When receiving the ACK frame in response to the SP-poll frame, the STAmay recognize that the AP cannot send a buffered frame. In such case,the first polled service period initiated by transmission of the SP-pollframe may be terminated. The STA sends out an ACK frame and enters intothe doze state (S1924).

Meanwhile, the STA enters into the awake state at the time indicated bythe polled SP interval field of the SP-poll frame (S1931) and obtains achannel access authority through contention (S1932).

When obtaining the channel access authority, the STA requests that theAP send a frame through transmission of the SP-poll frame (S1933). Asecond polled service period is initiated through transmission of theSP-poll frame.

The AP receives the SP-poll frame and may send at least one bufferedframe to the STA during the second polled service period (S1941 andS1942). In this example, the second polled service period is assumed tobe as long as the AP may send the buffered frame to the STA twice.

Even when the AP has more traffic buffered for the STA than the amountof traffic that may be treated by sending out the buffered frame twotimes, the AP cannot exceed two times in transmitting the bufferedframe. Accordingly, the AP needs to inform the STA that there is stilldata to be transmitted to the STA. This may be signaled through the MD(More Data) field of the frame control field in the frame.

The AP may conduct transmission with the MD field of the second bufferedframe sent in step S1942 set as 1 in order to signal the STA that datastill remains to be sent. The STA may be aware that the AP has for datato be transmitted by receiving the second buffered frame and identifyingthe MD field of the frame control field.

The STA sends an ACK frame to the AP as a response to acknowledgereception of the buffered frame received during the second polledservice period (S1943). If the second polled service period expires, itenters into the doze state (S1944).

The STA may request that a buffered frame be sent by sending an SP-pollframe again. For this, the STA enters into the awake state at the timeindicated by the polled SP interval field of the SP-poll frametransmitted in step S1933 (S1951) and obtains a channel access authoritythrough contention (S1952).

When obtaining the channel access authority, the STA requests that theAP send a buffered frame through the SP-poll frame (S1953). Through thetransmission of the SP-poll frame, a third polled service period isinitiated.

The AP may send a buffered frame to the STA during the initiated secondpolled service period an SIFS after receiving the SP-poll frame (S1954).At the time, the AP may send a remaining buffered frame that was nottransmitted during the second polled service period.

The STA sends an ACK frame in response to the buffered frame receivedfrom the AP (S1955) and enters into the doze state (S1956).

In the frame transmission and reception method according to FIG. 19, theAP sends an ACK frame during the first polled service period in responseto the STA's SP-poll frame. Accordingly, a deferred SP-pollmechanism-based frame transmission and reception method is conductedduring the first polled service period. During the second polled serviceperiod and third polled service period, the AP sends at least onebuffered frame in response to the SP-poll frame. Accordingly, animmediate SP-poll mechanism-based frame transmission and receptionmethod is performed during the second polled service period and thethird polled service period.

According to the frame transmission and reception method illustrated inFIGS. 17 to 19, the STA may receive a beacon frame from the AP duringmultiple counts of a polled service period, and enter into the dozestate between the polled service periods, so that power consumption maybe prevented. Further, the STA may receive at least one or more bufferedframes during one polled service period, thus enabling efficient datatransmission and reception. In addition, since the AP may send abuffered frame during a service period even without conducting RTS/CTSexchange in order to transmit a buffered frame, the frame transmissionand reception efficiency may be further enhanced.

When the STA obtains a buffered frame from the AP based on the frametransmission and reception method according to the above-describedembodiments, a device may be needed to prevent collision with framestransmitted and received by other STAs. For such purpose, other STAs mayconfigure an NAV (Network Allocation Vector) based on the SP-poll frametransmitted from the STA.

FIG. 20 is a view illustrating another example method of transmittingand receiving a frame according to an embodiment of the presentinvention. In the example illustrated in FIG. 20, it is assumed thatSTA1 and STA3 are located in the service coverage of the AP and thatSTA2 is located in the coverage of STA1.

Referring to FIG. 20, STA1 enters into the awake state in order toreceive a TIM element (S2010) and STA1 receives a TIM element (S2020).

When verifying that a bufferable frame is being buffered based on theTIM element, the STA transmits an SP-poll frame to the AP (S2030).

The AP may send the buffered frame to the STA during a polled serviceperiod initiated by transmission of the SP-poll frame (S2041 and S2042).

If the polled service period expires, STA1 sends an ACK frame to the AP(S2050) and enters into the doze state (S2060).

Since STA2 is positioned outside the service coverage of the AP, STA2cannot receive a frame transmitted from the AP. On the contrary, STA1 ispositioned within the coverage and thus may receive a frame transmittedby STA1. S12 may overhear the SP-poll frame transmitted from STA1(S2071). Accordingly, STA2 verifies the duration of the polled serviceperiod through the duration field of the SP-poll frame and may configurean NAV during the duration (S2072). As the NAV is configured by STA2,collision between STA1 and STA2 may be prevented.

STA3 is located in the service coverage of the AP and thus may receive aframe received by the AP. In contrast, since STA3 is off the coverage ofSTA1, STA3 cannot receive a frame transmitted from STA1. In this case,STA3 may overhear buffered frames transmitted from the AP (S2081 andS2082).

STA3 may configure an NAV based on the duration information contained inthe MAC header and/or preamble of the buffered frame transmitted fromthe AP (S2091 and S2092). Accordingly, collision between STA3 and AP maybe prevented.

In case the AP has a small amount of buffered traffic and thus theduration of the polled service period actually initiated by the STA islong, the AP may process all the buffered traffic through a small countof transmission of the buffered frame, whereas the STA may stayaccessing the channel during the duration of the polled service period.This is not good in light of efficiency of radio sources and powerconsumption efficiency of the STA. To prevent this, a need exists for amethod of enabling the STA to forcedly terminate the polled serviceperiod specified by the duration field of the SP-poll frame transmittedfrom the STA even when the polled service period is not terminated tothereby enhance efficiency-related capabilities.

FIG. 21 is a flowchart illustrating another example method oftransmitting and receiving a frame according to an embodiment of thepresent invention.

Referring to FIG. 21, STA1 switches from the doze state to the awakestate in order to receive a TIM element (S2111) and receives a TIMelement (S2112).

When identifying based on the TIM element that a bufferable frame isbeing buffered, STA1 obtains a channel access authority throughcontention (S2121) and sends a SP-poll frame to the AP (S2122). A polledservice period is initiated by the SP-poll frame transmitted by STA1.

STA2 is located outside the service coverage of the AP and thus cannotreceive a frame transmitted from the AP. In contrast, since STA2 ispositioned in the coverage of STA1, STA2 may receive a frame transmittedfrom STA1. STA2 may overhear a SP-poll frame transmitted from STA1(S2123). Accordingly, STA2 may identify the duration of the polledservice period through the duration field of the SP-poll frame and mayconfigure an NAV during the duration.

The AP receives the SP-poll frame and sends a buffered frame to the STAduring the initiated polled service period (S2131).

STA1 receives a buffered frame from the AP, and when identifying thatthe AP does not send any more buffered frame, transmits an ACK frame tothe AP as a response to acknowledge reception of the received bufferedframe (S2132). Thereafter, STA1 may broadcast a CF (Contention Free)-end(CF-end) frame in order to arbitrarily terminate the polled serviceperiod (S2133).

When arbitrarily terminating the polled service period by sending outthe CF-end frame, STA1 enters into the doze state (S2134).

Since STA3 is positioned in the service coverage of the AP, STA3 mayreceive a frame transmitted from the AP. On the contrary, since STA3 ispositioned outside the coverage of STA1, STA3 cannot receive a frametransmitted from STA1. In this case, STA3 may overhear a buffered frametransmitted from the AP (S2135).

STA3 may configure an NAV during the period when the buffered frame istransmitted based on the duration information contained in the MACheader and/or preamble of the buffered frame by the AP.

Since STA2 is located in the service coverage of STA1, STA2 may receivethe CF-end frame transmitted from STA1 (S2136). When receiving theCF-end frame, STA2 may terminate the NAV configured per the durationfield of the SP-poll frame if the reception of the CF-end frame iscomplete. Accordingly, STA2 may try to access the channel if the serviceperiod is actually ended.

By the frame transmission and reception method according to FIG. 21, theSTA may control a polled service period per the transmission state ofthe AP's buffered frame. This may prevent the phenomenon that as thepolled service period initiated by the SP-poll frame is unnecessarilymaintained, even when transmission of the buffered frame from the AP isnot actually required, the STA keeps holding a channel access authority,so that the channel is unnecessarily occupied. Further, other STAs thatare positioned in the STA and/or the AP may also acquire a channelaccess authority by adjusting the NAV according to a service periodactually adjusted. Accordingly, the overall throughput of the wirelessLAN system may be enhanced.

In the above-described various frame transmission and reception methods,the information related to the polled SP interval is signaled, includedin the polled SP interval field of the SP-poll frame transmitted fromthe STA. Meanwhile, in order for the AP and STA to share the polled SPinterval, a method for the AP to signal the STA with polled SPinterval-related information may be offered. For this, a polled SPinterval information element containing the polled SP interval-relatedinformation is provided.

FIG. 22 is a block diagram illustrating the format of a polled SPinterval information element according to an embodiment of the presentinvention.

Referring to FIG. 22, the polled SP interval information element 2200includes an element ID field 2210, a length field 2220, and a polled SPinterval field 2230.

The element ID field 2210 may be configured to indicate that theinformation element is a polled SP interval information element.

The length field 2220 may be configured to indicate the length of thepolled SP interval field 2230.

The polled SP interval field 2230 may be configured to indicate aninterval between after a specific polled service period is ended anduntil a next polled service period begins. Or, the polled SP intervalfield 2230 may be configured to indicate an interval betweentransmission of a specific SP-poll frame and transmission of asubsequent SP-poll frame.

The polled SP interval information element 2200 may be included in anassociation response frame and/or probe response frame transmitted fromthe AP. When receiving the association response frame or probe responseframe, the STA sends an SP-poll frame according to the polled SPinterval indicated by the polled SP interval information element 2200included and initiate a polled service period.

By a method of the AP signaling the STA with information on a polledservice period, another information element may be defined. A responsetime information element is suggested below as information element forsuch purpose.

FIG. 23 is a block diagram illustrating the format of a response timeinformation element according to an embodiment of the present invention.

Referring to FIG. 23, the response time information element 2300includes an element ID field 2310, a length field 2320, a frame typefield 2330, and a response time field 2340.

The element ID field 2310 may be configured to indicate that theinformation element is the response time information element 2300.

The length field 2320 may be configured to indicate the length of theresponse time field 2340 and the frame type field 2330 that are includedthereafter.

The frame type field 2330 may include a type subfield 2331 and a subtypefield 2332. The type subfield 2331 may indicate the type of a frame,i.e., whether a frame is a management frame, a control frame, and/or adata frame. The subtype field 2332 may indicate the subtype of each typeof frame.

The response time field 2340 may include information on an expectedresponse time for each frame type that is supposed to be transmitted bythe AP as a response to the SP-poll frame transmitted by the STA.

The information elements shown in FIGS. 22 and 23 may be transmitted,included in an association response frame during an associationprocedure between the AP and the STA or included in a probe responseframe during a scanning procedure. Signaling the information elementduring the association procedure and/or scanning procedure may be topromise a polled SP interval between the AP and the STA. Accordingly,the STA may send an SP-poll frame and then send again a SP-poll frameaccording to the polled SP interval indicated by the response timeinformation element and/or polled SP interval information element tothereby initiate a polled service period, and the AP may send at leastone buffered frame to the STA during the service period initiated by theSTA.

Meanwhile, the polled SP interval by the information elements shown inFIGS. 22 and 23 may be a basic interval scheduled between the STA andthe AP. That is, even though a polled SP interval has been scheduled bytransmission of the information elements through an associationprocedure and/or scanning procedure, in case the STA conductstransmission with polled SP interval-related information set as aspecific value included in the SP-poll frame, the STA and the AP maycarry out a frame transmission and reception procedure based on thepolled SP interval-related information indicated by the SP-poll frame.

Additionally, the polled SP interval-related information such as theinformation elements shown in FIGS. 22 and 23 may be transmitted by theAP, and may be thus signaled. The AP sends an ACK frame in response tothe STA's SP-poll frame.

The SP-poll frame may have the format shown in FIG. 16, and the durationof the polled service period initiated by the SP-poll frame may bespecified by the duration field of the SP-poll frame. On the contrary,in this embodiment, the SP-poll frame might not contain the polled SPinterval field.

The ACK frame may include polled SP interval-related information. By wayof example, the ACK frame may contain an information element having theformat shown in FIG. 22 or FIG. 23. In case the polled SP intervalinformation element shown in FIG. 22 is included, the polled SP intervalfield may indicate a time when after transmission of the ACK frame, theAP intends to initiate transmission of a buffered frame, i.e., a timewhen a polled service period is intended to be started. In case theresponse time information element shown in FIG. 23 is included, theresponse time field may indicate a time when after transmission of theACK frame, the AP intends to initiate transmission of a buffered frame,i.e., a time when a polled service period is intended to begin.According to the polled SP interval-related information, the frametransmission and reception method may be immediate SP-poll based ordeferred SP-poll based. Hereinafter, this is described in greater detailwith reference to drawings.

FIG. 24 is a view illustrating an example method of transmitting andreceiving a frame by an STA operating in a power save mode according toanother embodiment of the present invention.

Referring to FIG. 24, the STA switches from the doze state to the awakestate in order to receive a TIM element (S2410).

The STA receives a TIM element (S2420). The TIM element may betransmitted, included in a beacon frame. When receiving the TIM element,the terminal may determine whether a bufferable frame for itself isbeing buffered based on the AID of the STA and the bitmap sequence ofthe partial virtual bitmap field included in the TIM element.

When identifying that a bufferable frame is being buffered, the STAobtains a channel access authority through contention (S2430), and maysend a request for transmission of a buffered frame to the AP throughtransmission of an SP-poll frame (S2440).

When receiving the SP-poll frame from the STA, the AP may send at leastone PPDU to the STA in response to the SP-poll frame. In response to theSP-poll frame, the AP may transmit an ACK frame (S2450). The ACK framemay contain polled SP interval-related information. In this example, thepolled SP interval-related information may be configured to indicatethat the AP is supposed to transmit a buffered frame immediatelysubsequent to transmission of the ACK frame. In this case, by way ofexample, the polled SP interval field or response time field is set as 0in order to enable the AP to immediately send a buffered frame withoutconfiguring a polled SP interval.

The AP sends at least one buffered frame to the STA an SIFS aftertransmission of the ACK frame (S2461, S2462, and S2463). The AP may sendto the STA a plurality of PPDUs including at least one buffered frameand ACK frame during the polled service period initiated by the SP-pollframe.

The STA receives the ACK frame from the AP and may identify the timewhen the AP intends to send a buffered frame based on the includedpolled SP interval information. In the present example, since the polledSP interval information is set as 0, the STA may determine that the APis to send a buffered frame after the ACK frame. Accordingly, the STAmaintains the awake state after receiving the ACK frame, thereby waitingto receive a buffered frame.

When receiving the last buffered frame among the, at least one, bufferedframe transmitted from the AP, the STA may send an ACK frame as aresponse to acknowledge reception of the buffered frame (S2470). Or, theSTA may send an ACK frame in compliance with expiration of the durationof the polled service period indicated by the duration field of theSP-poll frame.

When sending out the ACK frame, the STA enters into the doze state(S2480).

FIG. 25 is a view illustrating another example method of transmittingand receiving a frame by an STA operating in a power save mode accordingto another embodiment of the present invention.

Referring to FIG. 25, the STA shifts from the doze state to the awakestate so as to receive a TIM element (S2511).

The STA receives a TIM element (S2512). The TIM element may betransmitted, included in a beacon frame. When receiving the TIM element,the terminal may determine whether a bufferable frame for itself isbeing buffered based on the AID of the STA and the bitmap sequence ofthe partial virtual bitmap field included in the TIM element.

When identifying that a bufferable frame is being buffered, the STAobtains a channel access authority through contention (S2521) and maysend a request for transmission of a buffered frame to the AP throughtransmission of an SP-poll frame (S2522). As the SP-poll frame istransmitted, a first polled service period may be initiated.

The AP may send at least one PPDU to the STA in response to the SP-pollframe from the AP. The AP may transmit an ACK frame in response to theSP-poll frame (S2523). The ACK frame may contain polled SPinterval-related information. In this example, the polled SPinterval-related information may indicate a time when the AP intends toinitiate transmission of a buffered frame to the STA separately from thepolled service period initiated by the SP-poll frame. As an example, thepolled SP interval field or response time field may be set as a valueindicating a time when the AP intends to initiate transmission of abuffered frame to the AP or a time when a new polled service period isintended to be initiated.

The STA receives the ACK frame and may be aware of the time when the APintends to transmit a buffered frame based on the included polled SPinterval information. Accordingly, the STA may enter into the doze stateafter receiving the ACK frame (S2524). In this case, the first polledservice period initiated by transmission of the SP-poll frame may beended.

The STA enters into the awake state at the time indicated by the polledSP interval information contained in the ACK frame (S2531) and acquiresa channel access authority through contention (S2532).

When obtaining the channel access authority, the STA requests that theAP send a frame through transmission of an SP-poll frame (S2533). Asecond polled service period is initiated through transmission of theSP-poll frame.

The AP may send one or more buffered frames to the STA during theinitiated second polled service period an SIFS after receiving theSP-poll frame (S2541, S2542, and S2543).

The duration of the second polled service period initiated by the STAtransmitting the SP-poll frame (S2533) may be specified as the durationof the polled period described above in connection with FIG. 24. Thatis, the polled duration may be ended by the AP sending a buffered framecontaining an EOSP field set as 1. Or, the second polled duration may bespecified by the duration of the duration field of the SP-poll frametransmitted by the STA in step S2533.

The STA receives at least one or more frames from the AP and sends anACK frame in response thereto (S2544). The ACK frame sent by the STA maybe a block ACK as a response to acknowledge reception of at least one ormore buffered frames. The STA, after sending out the ACK frame, mayenter into the doze state (S2550).

FIG. 26 is a block diagram illustrating a wireless device in which anembodiment of the present invention may be implemented.

Referring to FIG. 26, the wireless device 2600 includes a processor2610, a memory 2620, and a transceiver 2630.

The transceiver 2630 transmits and/or receives radio signals andimplements the physical layer of IEEE 802.11.

The processor 2610 is operatively coupled with the transceiver 2630 andmay be configured to transmit and receive a TIM element to determinewhether a bufferable frame for itself is being buffered. The processor2610 may be configured to transmit an SP-poll frame. The processor 2610may be configured to transmit and/or receive at least one buffered frameduring a service period initiated through the SP-poll frame. Theprocessor 2610 may be configured to switch between the awake stateand/or doze state depending on transmission and reception of a TIMelement and a buffered frame. The processor 2610 may be configured toestablish a polled service period during the course of an associationprocedure and/or scanning procedure. The processor 2610 may beconfigured to implement an embodiment of the present invention asdescribed above in connection with FIGS. 16 to 25.

The processor 2610 and/or the transceiver 2630 may include an ASIC(Application-Specific Integrated Circuit), other chipset, a logiccircuit, and/or a data processing device. When an embodiment isimplemented in software, the above-described schemes may be realized inmodules (processes or functions) for performing the above-describedoperations. The modules may be stored in the memory 2620 and may beexecuted by the processor 2610. The memory 2620 may be included in theprocessor 2610 or may be positioned outside the processor 2610 and maybe operatively coupled with the processor 2610 via various known means.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps. Furthermore, those skilled in the art will understand that thesteps shown in the flowcharts are not exclusive and may include othersteps or one or more steps of the flowcharts may be deleted withoutaffecting the scope of the present invention.

1-14. (canceled)
 15. A method for operating in a power save mode in awireless local area
 1. A method for operating in a power save mode in awireless local area network (WLAN), comprising: transmitting, by anaccess point (AP), a beacon frame including a traffic indication map(TIM) to a receiving station (STA), the TIM indicating whether there isa buffered unit (BU) for the receiving STA; receiving, by the AP, a pollframe from the receiving STA if the TIM indicates presence of the BU forthe receiving STA, the poll frame including a duration field set by thereceiving STA and an address field, wherein the duration field indicatesa service period during which at least one frame is transmitted to thereceiving STA from the AP and an ACK signal for the at least one frameis received by the AP, and wherein the address field includingidentification information of the receiving STA, and transmitting, bythe AP, the at least one frame to the receiving STA.
 16. The method ofclaim 15, wherein the at least one frame includes a field indicatingwhether the at least one frame is a last frame during the serviceperiod.
 17. The method of claim 15, wherein the duration field is usedby an unintended receiving station of the poll frame to set its networkallocation vector (NAV) timer.
 18. The method of claim 15, wherein theat least one frame is included a physical layer protocol data unit(PPDU) including a physical layer preamble and a medium access control(MAC) PDU.
 19. Access point (AP) in a wireless local area network(WLAN), comprising: a transceiver that receives and transmits radiosignals; and a processor coupled to the transceiver that: controls thetransceiver to transmit a beacon frame including a traffic indicationmap (TIM) to a receiving station (STA), the TIM indicating whether thereis a buffered unit (BU) for the receiving STA; controls the transceiverto receive a poll frame from the receiving STA if the TIM indicatespresence of the BU for the receiving STA, the poll frame including aduration field set by the receiving STA and an address field, whereinthe duration field indicates a service period during which at least oneframe is transmitted to the receiving STA from the AP and an ACK signalfor the at least one frame is received by the AP, and wherein theaddress field including identification information of the receiving STA,and controls the transceiver to transmit the at least one frame to thereceiving STA.
 20. The AP of claim 19, wherein the at least one frameincludes a field indicating whether the at least one frame is a lastframe during the service period.
 21. The AP of claim 19, wherein theduration field is used by an unintended receiving station of the pollframe to set its network allocation vector (NAV) timer.
 22. The AP ofclaim 19, wherein the at least one frame is included a physical layerprotocol data unit (PPDU) including a physical layer preamble and amedium access control (MAC) PDU.